Inhibitors of the rate-limiting enzyme of bilirubin formation are being studied in normal adults and in patients with hepatic dysfunction. The agents, Sn-protoporphyrin and Sn-mesoporphyrin are potent competitors of heme oxygenase, and have now been shown to decrease bilirubin production and plasma and biliary bilirubin levels in a number of animal models and in humans. The compounds are remarkably nontoxic and are expected to be useful for ameliorating neonatal jaundice, thereby diminishing the risk of bilirubin CNS toxicity.